Data from: Individual, population, and ecosystem effects of hypoxia on a dominant benthic bivalve in Chesapeake Bay

Hypoxia is an environmental stressor that affects abundance, biomass, diversity and ecosystem function of benthic assemblages worldwide, yet its collective impact at individual, population, and ecosystem levels has rarely been investigated. We examined the effects of hypoxia on the biomass-dominant clam, Macoma balthica, in the York and Rappahannock Rivers (Chesapeake Bay, USA). We (i) surveyed the M. balthica populations in both rivers in 2003 and 2004, (ii) determined the effects of low DO on M. balthica fecundity in a laboratory experiment, and (iii) employed a predator-exclusion field experiment to establish the effects of hypoxia and prey density on predation upon M. balthica. The resultant data were used to parameterize a matrix model, which was analyzed to define potential effects of hypoxia at the population level. In both rivers, hypoxia decreased individual clam growth and caused local extinction of populations. Hypoxia reduced egg production of M. balthica by 40% and increased protein investment per egg. In the predator-exclusion field experiment, hypoxia magnified predation rates three-fold and altered the functional response of predators to M. balthica from a stabilizing type III functional response to a destabilizing type II functional response. In a density-independent matrix model, hypoxia resulted in coupled source-sink metapopulation dynamics, with hypoxic areas acting as black-hole sinks. Increases in the spatial and temporal extent of hypoxia caused the populations to decline toward extinction. In a second model that incorporated density-dependence, under mild hypoxic conditions trophic transfer from M. balthica to predators increased, but at increased spatial or temporal extent of hypoxia trophic transfer decreased. The major decline in trophic transfer to predators under severe hypoxia resulted from diversion of M. balthica biomass into the microbial loop. Our model predicted that there are multiple stable states for M. balthica populations (high and very low densities), such that the saddle point (threshold at which the population switches from one state to the other) increased and resilience decreased with the spatial extent of hypoxia. We underscore how effects of a stressor at the individual level can combine to have substantial population and ecosystem-level effects.低氧（hypoxia）作为一种全球性环境胁迫因子，会影响全球范围内底栖生物群落的丰度、生物量、多样性及生态系统功能，但目前极少有研究同时探究其在个体、种群及生态系统层面的综合影响。本研究以美国切萨皮克湾约克河与拉帕汉诺克河中的生物量优势种——白樱蛤（Macoma balthica）为研究对象，探究了低氧对其产生的生态效应。我们开展了三项工作：（i）于2003年与2004年对两条河流中的白樱蛤种群进行了野外调查；（ii）通过室内实验探明了低溶解氧（dissolved oxygen, DO）对白樱蛤繁殖力的影响；（iii）借助野外捕食者排除实验，明确了低氧与猎物密度对白樱蛤被捕食压力的调控作用。本研究所得实验数据被用于对矩阵模型（matrix model）进行参数化，并通过模型分析界定了低氧在种群层面的潜在影响。研究结果显示，在两条河流中，低氧均抑制了白樱蛤的个体生长，并导致其种群局部灭绝；低氧使白樱蛤的产卵量降低40%，同时提升了单枚卵的蛋白质投入量。在野外捕食者排除实验中，低氧将捕食率提升了三倍，并将捕食者对白樱蛤的功能响应（functional response）从具有稳定调控作用的III型功能响应（type III functional response）转变为具有破坏作用的II型功能响应（type II functional response）。在密度无关矩阵模型中，低氧引发了源-汇复合种群动态（source-sink metapopulation dynamics）的耦合效应，低氧区域则扮演了“黑洞汇”的角色。低氧的空间与时间覆盖范围扩大，会导致白樱蛤种群逐步走向灭绝。在另一项纳入密度依赖效应的模型中，轻度低氧条件下，白樱蛤向捕食者的营养传递效率有所提升，但当低氧的空间或时间覆盖范围进一步扩大时，营养传递效率反而出现下降。严重低氧条件下营养传递效率的大幅下降，源于白樱蛤的生物量被分流至微生物环（microbial loop）中。本模型预测，白樱蛤种群存在两种稳定状态（高密度与极低密度），鞍点（saddle point，即种群从一种状态切换至另一种状态的阈值）会随低氧空间覆盖范围的扩大而升高，种群恢复力则随之降低。本研究凸显了单一胁迫因子在个体层面的效应，如何通过协同作用进而对种群及生态系统产生显著影响。